1. Field of the Invention
The present invention relates to a jitter amplifier, a jitter amplification method, an electronic device, a test apparatus and a testing method. More specifically, the invention relates to a jitter amplifier for amplifying or attenuating a jitter component contained in an input signal and to an electronic device and a test apparatus having such jitter amplifier
2. Related Art
Conventionally, there has been known a PLL (Phase Locked Loop) as a circuit for outputting a jitter component contained in an input signal by changing it by a predetermined gain G. However, an original purpose of use of the PLL is to generate an oscillating signal synchronized with a reference signal and is not to change and output the jitter component
The PLL has a phase detector, a loop filter, a voltage controlled oscillator and a frequency divider. The phase detector compares phase of a predetermined reference signal (input signal) with that of an oscillating signal and outputs a control signal corresponding to their phase difference.
The loop filter passes a predetermined frequency component of the control signal. The voltage controlled oscillator generates the oscillating signal having frequency corresponding to voltage level of the control signal passed by the loop filter. This oscillating signal is fed back to the phase detector via the frequency divider. Having such arrangement, the PLL generates the oscillating signal synchronized with the reference signal.
Here, a transfer function of the phase detector will be denoted as Kd, a transfer function of the loop filter as F(s) and a transfer function of the voltage controlled oscillator as K0/s and a frequency dividing ratio of the frequency divider is assumed to be 1. In this case, an open-loop transfer function HOPEN(s) of the PLL may be expressed by Equation 1 as follows:
                                          H            OPEN                    ⁡                      (            s            )                          =                              Kd            ·                          F              ⁡                              (                s                )                                      ·                          K              0                                s                                    Eq        .                                  ⁢        1            
Further, a closed-loop transfer function HCLOSED(s) may be expressed by using the open-loop transfer function by Equation 2 as follows:
                                          H            CLOSED                    ⁡                      (            s            )                          =                                            H              OPEN                        ⁡                          (              s              )                                            1            +                                          H                OPEN                            ⁡                              (                s                )                                                                        Eq        .                                  ⁢        2            
From the above, a gain G(ω) of the PLL may be expressed by Equation 3 in a control zone as follows:
                              G          ⁡                      (            ω            )                          =                              ±                                                                          H                  CLOSED                                ⁡                                  (                  jω                  )                                                                            =                      ±                                                                                              H                    OPEN                                    ⁡                                      (                    jω                    )                                                                    1                  +                                                            H                      OPEN                                        ⁡                                          (                      jω                      )                                                                                                                                            Eq        .                                  ⁢        3            
Here, ω is angular frequency.
Accordingly, the gain G(ω) of the PLL is in a range of 1 or less and −1 or more.
The PLL is designed so that the gain G(ω) is almost zeroed in general in order to reduce jitter in the oscillating signal. However, the gain G(ω) must be almost 1 in a low offset frequency range in order to synchronize the phase of the oscillating signal with the reference signal. Therefore, it is unable to attenuate the jitter in the low offset frequency even if the PLL is used as a jitter attenuator.
Still more, because the gain G(ω) of the PLL is in the range of 1 or less and −1 or more as described above, it is unable to amplify the jitter component of the reference signal. That is, the PLL is incapable of functioning as a jitter amplifier.